Themes: Optimize responsive viewport settings

* Update: Add to existing viewport settings (instead of completely replacing viewport settings)
Merge branch 'opt_viewport' into develop
i18n: Enable Admin messages
Cleanup: Remove legacy hooks
Finalize label text
i18n: Update POT file
Remove legacy options

* validate_links
* ui_enabled_desc
* ui_enabled_caption
* ui_caption_src
Load client output in footer (instead of header)

* Libs: Core, View, Admin
* Context loaded after libs
* Components loaded after libs: Content Handlers, Themes, Template Tags

Utilities
* Add: Standard priorities helper method (`priority()`)
Merge branch 'load_client_code' into develop
Update readme
Update plugin metadata and readme

* Prep for release on WP.org plugins directory
Update version and URI for release

* URI points to release notes
Remove debug code
Merge branch 'release-2.0rc2'